This invention relates to methods and apparatus for fluid catalytic cracking of hydrocarbon oils. More particularly this invention relates to methods wherein catalyst and hydrocarbon vapor discharge from one or more riser reactors into a reaction vessel containing a selected weight of dense phase fluidized catalyst. Hydrocarbon conversion in the reaction vessel, in addition to that obtained within the riser reactors, is controlled by increasing and decreasing the volume of dense phase catalyst bed such that contact of hydrocarbon vapor with catalyst is increased or decreased to obtain the desired degree of cracking.
In fluid catalytic cracking processes, hydrocarbon oils are contacted in one or more reaction zones with cracking catalyst under conditions such that a portion of the hydrocarbon oils are converted into lower boiling products. During the hydrocarbon conversion, coke is deposited upon the catalyst. After contact with the hydrocarbon charge, coke contaminated catalyst is removed from the reaction zone, from which it may be transferred to a stripping zone. In the stripping zone, volatile hydrocarbons entrained within, or occluded upon, the catalyst are vaporized employing a stripping vapor, such as steam. Stripping vapors and vaporized hydrocarbons are transferred from the stripping zone into the reaction zone from which they are subsequently recovered as components of the reaction zone hydrocarbon product. Stripped catalyst from the stripping zone is transferred to a regeneration zone wherein at least a portion of the coke is removed by burning with an oxygen containing gas, thus regenerating and restoring catalytic activity to the catalyst. Regenerated catalyst from the regeneration zone is returned to the reaction zone for contact with additional hydrocarbon oils.
According to one method for cracking hydrocarbons, regenerated catalyst and hydrocarbon charge are combined near the bottom of an elongated riser reactor conduit under catalytic cracking conditions. The resulting catalyst-hydrocarbon vapor mixture flows upward in the riser as a dilute suspension of catalyst in hydrocarbon vapor, and the mixture is subsequently discharged into a reaction vessel. In the reaction vessel, hydrocarbon vapors and catalyst separate, forming a dense phase fluidized catalyst bed and a hydrocarbon vapor phase containing minor amounts of suspended catalyst. In this method of cracking hydrocarbon oils, one or more risers may be employed. For instance, in U.S. Pat. Nos. 3,394,076 and 3,433,733, fluid catalytic cracking methods are described when two riser reactors are employed. According to the methods of the referenced patents, fresh hydrocarbon charge in a first riser is combined with regenerated catalyst for reaction therein, and in the second riser recycle oil, comprising relatively high boiling components obtained from the cracked hydrocarbon products of the catalytic cracking reaction, is combined with regenerated catalyst and subjected to additional cracking. Both risers discharge into the reaction vessels wherein a dense phase bed of catalyst is maintained in a fluidized state by the passage of hydrocarbon vapors and fluidization vapors therethrough. Above the dense phase there is a dilute phase which has a catalyst concentration of only about 0.15 to about 0.75 pounds per cubic foot. Hydrocarbon vapors entering the reaction zone via the riser or risers provided a substantial proportion of the vapors required to maintain the dense phase catalyst bed in a fluidized state. Additional fluidization vapors, which may comprise primary stripping steam or other gases, are added near the bottom of the dense phase bed for separating a portion of occluded hydrocarbons from the catalyst and maintaining the dense phase bed in a fluidized state. Catalyst is continuously discharged from the risers into the reaction vessel, and is continuously withdrawn drom the dense phase fluidized bed in order to maintain a desired volume inventory of catalyst within the reaction vessel. Generally, bulk density of the fluidized dense phase catalyst bed is maintained about constant in the range of about 38-35 lb/cu.ft. by addition of fluidization vapors near the bottom of the reaction vessel. The proportion of hydrocarbon conversion obtained in the reaction vessel is determined by the degree of contact of hydrocarbon vapors with catalyst in the dense phase fluidized bed. Thus, according to these prior art methods the volume of dense phase is decreased or increased to increase or decrease hydrocarbon conversion within the reaction vessel.
According to the methods of the prior art, an inventory of dense phase catalyst within the reaction vessel is maintained at a selected value by measuring the pressure differential between a point above the upper surface of the dense phase fluidized bed and a second point below said upper surface and controlling the rate at which catalyst is withdrawn from the dense phase fluidized bed to maintain the measured pressure differential at a preselected value. This method for controlling the dense phase catalyst inventory measures weight of catalyst in the dense phase fluidized bed. An increase or decrease in dense phase catalyst inventory is measured by an increase or decrease in the measured pressure differential, since it is not common practice to vary the catalyst dense phase bulk density as a control parameter. Thus, for control of hydrocarbon cracking within the reaction vessel, the weight inventory of catalyst is increased or decreased to obtain increased or decreased contact of hydrocarbon with catalyst therein. Changing the weight inventory of catalyst within the reaction vessel, to obtain a desired degree of hydrocarbon conversion within the reaction vessel, requires that substantial amounts of catalyst be transferred into or out of the reaction vessel.
Coke deposited upon the catalyst within the dense phase fluidized bed in the reaction vessel adversely affects the catalytic activity of the catalyst. Also, even where primary stripping steam is provided in the reaction vessel, such dense phase catalyst has appreciable amounts of volatile relatively high boiling hydrocarbon liquids occluded thereon. Thus, catalyst removed from the reaction vessel is commonly stripped of volatile hydrocarbons in a stripping zone, then passed into a regeneration zone wherein coke and any other combustible materials are removed by burning with an oxygen containing gas, such as air. The removal of coke and combustible materials from the catalyst restores its catalytic activity, thereby making it suitable for further use in the fluidized catalytic cracking process. Accordingly, catalyst withdrawn from the reaction vessel is passed through a stripping zone wherein it is intimately contacted with a stripping vapor, preferably steam. The major portion of volatile hydrocarbons occluded upon the catalyst are thereby vaporized and stripped from the catalyst. Stripping vapors and vaporized hydrocarbons are commonly passed into the reaction vessel at a point above the upper surface of the dense phase fluid catalyst bed, via a stripper vent line. By employing this means for handling stripper vapors and vaporized hydrocarbons, the vaporized hydrocarbons may be conveniently recovered along the hydrocarbon products from the reaction vessel. Additionally, by passing such vapors from the secondary stripping zone into the reaction vessel above the fluidized catalyst bed, the pressure differential between the reaction vessel and the stripping zone is limited to the pressure through the stripper vent line. This pressure drop is small and may be controlled by properly sizing the cross-sectional area of the stripper vent line. Therefore, since the pressure differential between the reaction zone and the stripping zone is small, removal of catalyst from the reaction vessel into the stripping zone is not complicated by large differences in pressures between the two zones.